Method of monitoring hidden coal-rock interface and transducer realizing this method

ABSTRACT

A method for monitoring a hidden coal-rock interface, comprising the steps of irradiating the medium (M) being monitored from a gamma-ray source (1) and registering by a detector (2) the intensity of backward scattered radiation at a distance (A) from the surface of the medium (M) and at a distance (B) from the radiation source (B). Two zones (Z 1  and Z 2 ) are formed at the detector (2) for reception of backward scattered radiation, differently spaced from the source (1), so that the intensity of backward scattered radiation received by the zone (Z 1 ) closer to the source (1) diminished with the distance (A) from the detector (2) to the medium (M), and grows when received by the zone (Z 2 ) far from the source (1). The intensities of backward scattered radiation of both zones (Z 1  and Z 2 ) are summed up to obtain total intensity invariant with respect to variations of the distance (A) between the detector (2) and the medium (M). A transducer realizing the above method comprises a housing accomodating the gamma-ray source (1) and backward scattered radiation detector (2) shielded by a screen (3) provided with ports at different distances from the source (1). The area of the port farther from the source (1) is greater than that of the nearer port; the distance (B) from the gamma-ray source (1) to the detector (2) does not exceed a preset maximum distance (A 2 ) from the detector (2) to the medium (M) being monitored.

FIELD OF THE INVENTION

The present invention relates to devices for automatic steering ofcoal-winning machines according to seam hypsometry and, moreparticularly, it relates to a method of monotoring hidden coal-rockinterface and to a transducer realizing this method and intended tominitor hidden coal-rock interface by registering the intensity of backscattered gamma-radiation.

The present invention can be emploted to utmost advantage fornon-contact monitoring of coal-rock interface for automation offront-loading units, cutter-loaders of any size, units for chamberextraction, and other machines operated for full-seam extraction.

The invention can also be employed in geological exploration of bedrockwherever it occurs: on the surface, in mine workings, or in boreholes.

The invention can be further used for measuring the density of soil,rock, and various construction materials in situations of one-sidedaccessibility to areas being explored. It can be also embodied in mobiledensity meters for measuring the desity of rock along geologicalprofiles, sections, and routes.

The invention can be also employed for monitoring the thickness ofmaterial in an environment of one-sided access to a medium beinginvestigated, with a variable air gap between the transducer and medium,e.g. a layer of material carried by a running conveyer belt, or elsemoving rolled strip or sheet.

PRIOR ART

There is known a transducer of a hidden coal-rock interface developed inGreat Britain (see D. Hartlet "Automatic steering of cutter-loader inWallstenton Mine", Mining Engineer, 1971, Vol. 130, No. 124, P. 221)comprising a housing accomodating a gamma-ray source and a detector ofback scattered gamma-radiation, spaced from the center of the source.The method of monitoring a hidden coal-rock interface embodied in thisknown transmitter includes irradiating the medium being monitored fromthe gamma-ray source, registering backward scattered gamma-radiation bythe detector, and determining the hidden coal-rock interface from theintensity of backward scattered gamma-radiation thus detected. To ensurereliable performance of the device, the transmitter is urged against theroof of a mine working by a double-acting jack.

To provide favourable conditions for moving the transmitter along theroof, the transmitter is provided with tail portions at the sides of thesource and of the detector. The total length of the transmitter with thetail portion is 120 cm.

The transmitter incorporates a cavity detector which disconnects theautomatic control system of the cutter-loader when an air gap occursbetween the transmitter and the medium being monitored, as the presenceof an air gap results in the transmitter sending false signals causingmalfunctioning of the entire cutter-loader control system.

Normal performance of the known transmitter is dependent on its reliableengagement with the rock body being monitored, which necessitates theemployment of complicated hold-down and urging devices, as well as theincorporation of a cavity-detecting device. Furthermore, the reliabilityof this contact-type transmitter is affected by its operation incontinuous friction-type engagement which is difficult to maintain withadequate dependability from a moving machine. As it has been alreadymentioned, should an air gap occur between teh transmitter and themedium being monitored, the credibility of data sent out by thetransmitter is impaired.

The transmitter is rather large and cannot be built inot the screwconveyor structure of the cutter-loader to reduce to zero the transportlag. On account of the considerable dimensions of the known transmitter,it is positioned at the seam top behind the screw conveyor, which causesa transport lag between the point of application of the control action(accounting for a varying relief of the seam top) and the point of themonitoring of this action. The transport lag impairs the effectivenessof both monitoring and control.

Attempts to operate the known transmitter for monitoring the bottom of aseam have so far been unsuccessful on account of its high susceptibilityto variations of the air gap between the transmitter and the mediumbeing monitored.

There is further known a tansmitter for monitoring a hidden coal-rockinterface, based on density measurements (see U.S. Pat. No. 3,321,625),comprising a radiation source with Cesium¹³⁷ accommodated in acollimator, and two detectors. The transmitter incorporates a springdevice urging it against the surface being monitored and also serving asa borehole caliper. The bottom detector (a gas-discharge counter) is setclose to the housing at a small spacing (17.8 cm) from the radiationsource, while the top detector (a scintillation counter) is mountedinside the collimator at a 40.6 cm distance from the source. The bottomdetector is connected to an intensity meter, and the top counter isconnected through an amplitude analyzer to its own intensity meter. Thetwo intensity meters are connected with a computation device determiningthe logarithm of the ratio of the signals coming from the detectors. Toabsorb soft (low-energy) gamma radiation of energy below 50 keV, asilver or cadmium screen is interposed between the scatterer and theremote detector, the latter being intended for measuring density valueswith eventual compensation for the influence of clay crust present inthe borehole intermediate the transmitter and the surface beingmonitored; however, the output signal of the detector is highlysusceptible to a varying air gap between the transmitter and the mediumbeing monitored. The last-mentioned fact necessitates highly reliableurging of the transmitter against the medium being monitored, so thatdespite the incorporation of sophisticated urging devices and cavitymonitors, the reliability of the performance of the transmitter isimpaired. Relatively great dimensions of the transmitter would not allowto accommodate it directly within the cutting tool along the cuttingline of its teeth, which, as it has been already explained hereinabovein connection with the coal-rock interface monitor, impairs theeffectivemenss of the monitoring and control operation.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a simple method ofnon-contact monitoring of a hidden coal-rock interface, and to embodythis method in a compact transmitter-monitor of a coal-rock interface,invariant with respect to air gap fluctuations, which should enhance theperformance reliability of the transmitter in operation both at the topand bottom of a seam, thus providing for efficient monitoring andcontrol.

With this object in view, the essence of the invention resides in amethod of monitoring a hidden coal-rock interface, including irradiatinga medium being monitored from a gamma-ray source, registering backwardscattered gamma radiation by a detector, and determining the hiddencaol-rock interface from the intensity of backward scattered radiation,in which method, in accordance with the invention, the registration ofbackward scattered gamma radiation is performed at a distance from thesurface of the medium being monitored and at a distance from thegamma-ray source, not exceeding a preset value of maximum spacing of thedetector from the medium being monitored, there being formed at thedetector two zones of reception of backward scattered radiation,differently spaced from the radiation source, so that the intensity ofbackward scattered raditaion received by the zone closer to the sourcediminishes with an increasing spacing of the detector from the mediumbeing monitored, while the intensity of backward scattered radiationreceived by the zone more remote from the source grows, the intensitiesof backward scattered radiation received at the two zones being summedup to obtain the summary intensity of backward scattered radiationinvariant with respect to a varying spacing of the detector from themedium being monitored.

The disclosed method provides for conducting non-contact monitoring of ahidden coal-rock interface, while ensuring invariance of receivedbackward scattered gamma-radiation with respect to a varying spacing ofthe detector from the medium being monitored, so that the reliabilityand efficiency of the monitoring operation are enhanced.

It is expedient that invariance of the summary intensity of backwardscattered radiation with respect to a varying spacing of the detectorfrom the medium being monitored within a preset maximum value of thisspacing should be provided for by varying the surface area of thereception zones fo the detector.

A variation of the surface area of the reception or responsive zones isthe simplest way of realizing the diminshing and growing character ofthe intensities of received backward scattered gamma radiation inresponse to a varying spacing of the detector from the medium beingmonitored.

It is also possible to additionally ensure invariance of the summaryintensity of received backward scattered gamma radiation with respect toa varying spacing of the detector from the medium being monitored bydisplacing the radiation source and/or the detector in a vertical plane.

Such displacement of the source and/or detector permits a high accuracyin avoiding the influence of the spacing of the detector from the mediumbeing monitored within a preset range of its variation.

It is alternatively possible to additionally ensure invariance of thetotal intensity of received backward scattered radiation with respect toa varying spacing of the detector from the medium being monitored byvarying the angle of incidence of gamma-rays from the source on themedium being monitored.

Such variation of the incidence angle of gamma-rays allows to extend therange of no response of the total intensity of received backwardscattered gamma radiation to a varying spacing of the detector from themedium being monitored when this radiation is measured at a minimumspacing from the gamma-ray source.

It is expedient tht a transmitter of a hidden coal-rock interface,comprising a housing accommodating a gamma-ray source and a detector ofbackward scattered gamma radiation spaced from the source and enclosedin a screen attenuating backward scattered radiation should have, inaccordance with the invention, ports made in the screen at differentdistances from the gamma-ray source, the area of the port more remotefrom the source being greater than the area of the port less remote fromthe source, and the spacing of the source from the detector notexceeding a preset value of the maximum spacing of the detector from amedium being monitored.

This design of a transmitter provide for substantially reducing itsdimensions, while at the same time ensuring invariance of its outputsignal with respect to a varying air gap between the transmitter and themedium being monitored, thus enhancing the reliability of the monitoringoperation.

It is expedient that the ports should have their respective areasensuring that the sum of the intensities of bakcward scattered radiationreceived by the detector through the respective ports is substantiallypermanent within a preset range of variation of the air gap, to enhancethe accuracy of the monitoring of a coal-rock interface.

It is reasonable to select the material and thickness of the screen froma condition:

    2≦exp (μρd) ≦300,

where

μ is the mass coefficient of attenuation of gamma radiation by thescreen, cm² /g;

ρ is the density of the material of the screen, g/cm³ ;

d is the thickness of the screen, cm.

the above condition offers the simplest approach to the selection of therequired material and thickness of the radiation-filtering screen,providing for a maximum attainable value of the intensity of backwardscattered gamma radiation, invariant with respect to a varying air gapbetween the transducer and the medium being monitored.

It is expedient that one end of the housing should be cut at an angle of25° to 50° to the base of the housing, the housing having an openingmade therein normally to the cutting plane, accommodating the gamma-raysource, the detector including a holder with a plurality ofgas-discharge counters arranged normally to the base of the housing andconnected in parallel with one another.

This design of a transmitter, while retaining its compact size, providesfor sharply extending its range of no response to a varying air gapbetween the transducer and the medium being monitored.

It can be expedient to mount the radiation source and/or detector in thehousing adjustably in a vertical plane.

With the source and/or detector thus mounted, the accuracy of ensuringinvariance of the output signal of the transducer with respect to avarying spacing of the transducer from the medium being monitored withina required range of this variation is enhanced, while the compact sizeof the transmitter is retained.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be betterunderstood from the following description of its embodiment, withreference being made to the accompanying drawings, wherein:

FIG. 1 schematically illustrates the essence of a method of monitoring ahidden coal-rock interface in accordance with the invention;

FIG. 2 is a diagram of intensity of back scattered gamma radiationagainst the spacing of the detector from the medium being monitored;

FIG. 3 schematically illustrates a longitudinally sectional view of atransmitter of a hidden coal-rock interface embodying the invention, inits first version;

FIG. 4 shows the transmitter of FIG. 3, viewed from the side of themedium being monitored;

FIG. 5 schematically illustrates a longitudinally sectional view of atransmitter of a hidden coal-rock interface embodying the invention, inits second version;

FIG. 6 shows the transmitter of FIG. 5, viewed from the side of themedium being monitored;

FIG. 7 is a diagram of the intensity of back scattered gamma radiationagainst the thickness of a coal band overlying the rock;

FIG. 8 shows a diagram of the intensity of back scattered radiationagainst the air gap between the transducer and the medium beingmonitored, the transducer being placed ont he conveyer-screw workingmembers;

FIG. 9 is a diagram of the intensity of back scattered gamma radiationagainst the air gap between the transducer and the medium beingmonitored, with the transducer built into the supporting surfaces of theworking-face equipment (loading boards, bases of power support units,conveyer chutes, support skids, etc.).

BEST MODE FOR CARRYING OUT THE INVENTION

The essence of the present invention is illustrated by the schematicdrawing of FIG. 1 and the diagram of FIG. 2.

In the description hereinbelow, like parts will be referred to by likenumerals and symbols.

A medium being monitored is irradiated from a gamma-ray source 1, andbackward scattered radiation is registered by a detector 2. In a generalcase being presently described, the medium M bein monitored is a layerof coal of certain thickness overlying rock in the form of asemi-infinite layer.

With the thickness of the coal layer overlying the rock increasing, theintensity of backward scattered gamma radiation grows, and with thisthickness decreasing, the intensity diminishes. Thus, the value ofintensity is considered representative of the thickness of coaloverlying the rock, in which way the hidden coal-rock interface islocated. The intensity of backward scattered gamma radiation also varieswith a varying spacing A₁ of the detector 2 from the surface of themedium M being monitored, which impairs the credibility of themonitoring of a coal-rock interface.

To ensure that the intensity of backward scattered gamma radiation isunaffected by or invariant with respect to a varying spacing A₁, theregistering of backward scattered radiation is carried out at a spacingfrom the surface of the medium M being monitored and at a distance Bfrom the source 1 of gamma-rays to the detector 2, the value of thisdistance B not exceeding the maximum predetermined spacing A₂ of thedetector 2 from the medium M being monitored. There are formed at thedetector 2, e.g. with the aid of a screen 3 of a predeterminedthickness, two zones of reception differently spaced from the source 1:respectively, an adjacent zone Z₁ and a remote zone Z₂, the intensitiesof backward scattered radiation received in the two zones Z₁ and Z₂being summed up to obtain the summary intensity invariant with respectto the spacing of the detector from the medium being monitored, thisvalue being used to locate the hidden interface between coal and rock.With the spacing of the transmitter from the medium M being monitoredvarying from A₁ to A₂, the intensity I_(Z1) of backward scattered gammaradiation registered by the detector 2 as received at the zone Z₁ closerto the source 1 would also vary, the character of its variation beingdefined by the following factors.

First, the value of intensity I_(Z1) would grow owing to the growingarea of two-way source/detector visibility in the medium being monitored(this area C₂ of two-way visibility being greater than the area C₁).

Second, the value of intensity I_(Z1) would diminish due to thereduction of the working volume of the detector 2 registering backwardscattered gamma radiation received at the close area Z₁.

It can be clearly seen in FIG. 1 that with the spacing from themonitored medium M equalling A₂ and with radiation being scattered by anelementary volume "dv", the working volume of the detector 2 limited bythe ray 4 is less than the working volume of the detector 2 limited bythe ray 5 when the spacing from the monitored medium M equals A₁.Employing the screen 3, the reception zone Z₁ is so formed that thesecond-mentioned factor should dominate (over the first-mentioned one),and the intensity I_(Z1) should diminsh with an increasing spacing fromA₁ to A₂ (curve I in FIG. 2).

Plotted in FIG. 2 are curves illustrating the dependence of theintensity of backward scattered gamma radiation on the spacing of thedetector from the medium being monitored, the Y-axis showing theintensity I, pulses per second, and X-axis representing the spacing A,mm. Curve I in the diagram is the dependence I_(Z1) =f(A), i.e.variation of the intensity of backward scattered gamma radiationreceived by the zone Z₁ as a function of the spacing from the mediumbeing monitored.

Curve II in the same diagram is the dependence I_(Z2) =f(A), i.e.variation of the intensity of backward scattered gamma radiationreceived by the zone Z₂ as a function of the spacing from the mediumbeing monitored.

Curve III represents a function (I_(Z1) +I_(Z2))=f(A), i.e. variation ofthe summary intensity of backward scattered gamma radiation receivedsimultaneously by both zones Z₁ and Z₂ as a dunction of a varyingspacing from the medium being monitored.

Factors similar to those described above influence the intensity I_(Z2)of backward scattered gamma radiation received by the zone Z₂ remotefrom the source 1, which is formed by correspondingly selecting itsarea. However, with the spacing A₁ growing toward the predeterminedmaximum value A₂ of the spacing of the detector 2 from the medium Mbeing monitored, the value of intensity I_(Z2) would presistently grown(curve II in FIG. 2) owing to the increasing area of two-waysource/detector visibility at the monitored medium M, and to the growingworking volume of the detector 2 responding to backward scattered gammaradiation. The working volume of the detector 2 (FIG. 1) limited by theray 6 is obviously greater than the working volume limited by the ray 7.

Thus, as it can be seen in FIG. 2, the summing up of the decresingintensity I_(Z1) (curve I) and growing intensity I_(Z2) (curve II)yields the summary intensity I_(Z1) +I_(Z2) (curve III) of backwardscattered gamma radiation received at the two zones, which is invariantwith respect to the spacing of the detector 2 from the monitored mediumM varying from A₁ to A₂.

Invariance of the summary intensity of backward scattered gammaradiation with respect to a varying spacing of the detector 2 from themonitored medium M can be also attained by displacing either thegamma-ray source 1 or the detector 2, or both, in a vertical plane.

Depending on the extent of the abovementioned displacement, theinversion point of the function I_(Z1) =f(A) (curve I in FIG. 2) and theinversion point of the function I_(Z2) =f(A) are either brought closerto each other or moved apart, so as to ensure invariance of the summaryintensity (I_(Z1) +I_(Z2)) (curve III) with respect to a varying spacingfrom A₁ to A₂ of the detector 2 from the monitored medium M.

In the embodiment schematically illustrated in FIG. 1 the angle ofincidence of gamma rays from the source 1 on the monitored medium M is90° (α=90° ). According to the disclosed method, invariance of thesummary intensity of backward scattered gamma radiation with respect toa varying spacing of the detector 2 from the monitored medium M can bealso attained by varying the angle of incidence of gamma rays from thesource 1 on the medium M being monitored. Thus, by decreasing the angleof incidence (α<90° ), the inversion point of the function I_(Z2) =f(A)(curve II in FIG. 2) can be moved into the range of spacings A >A₂,which expands the range of invariance of the summary intensity (I_(Z1)+I_(Z2)) with respect to a varying distance or spacing of the detector 2from the medium M being monitored.

The disclosed method provides for non-contact monitoring of a coal-rockinterface at the minimum spacing B of the source 1 from the detector 2under conditions of a varying spacing of the detector 2 from the mediumM being monitored, while ensuring invariance of the registered intensityof backward scattered gamma radiation with respect to a variation of thelast-mentioned spacing within an adequately broad range from A₁ to A ₂,or even broader.

A method according to the invention can be performed by a transducer formonitoring a hidden coal-rock interface schematically illustrated inFIGS 3 and 4.

The transducer comprises a housing 8 having mounted therein a gamma-raysource 1 set at a spacing B from a detector 2 enclosed in a screen 3.The gamma-ray source 1 and detector 2 are both mounted for adjustment ina vertical plane. The screen 3 has ports made therethrough, differentlyspaced from the gamma-ray source 1: a port 9 of an area S₁ closer to thesource 1 and a port 10 of an area S₂ remote from the source 1, the areaS₂ of the port 10 more remote from the source 1 being greater than thearea S₁ of the port 9 closer to the source 1. The spacing B of thegamma-ray source 1 and detector 2 does not exceed a predeterminedmaximum value of the spacing of the detector 2 from the medium M beingmonitored. Under real operating conditions, of practical importance isthe value of the spacing of the transmitter from the monitored medium,which will be hereinafter referred to as an air gap "h"between thetransmitter and the monitored medium M.

The monitored medium M represented in the embodiment being described bya band of coal of a thickness H overlying the parent rock will bereferred to hereinafter as the monitored coal-rock medium.

The shape of the ports 9 and 10 shown in FIG. 4 is square for the utmostsimplicity of making them; however, the ports 9 and 10 may havedifferent shapes, e.g. oval or trapezoidal, provided that their areasare not equal, i.e. S₂ >S₁.

The housing 8 of the transducer being described additionallyaccommodates componenets commonly incorporated in transducers of thisgeneral type, such as a converter for power supply of the detector 2 anda pulsed amplifier for transmitting pulsed signals from the transmittervia a cable to secondary measuring apparatus (the converter andamplifier not shown in the appended drawings for clarity sake).

The housing 8 of the transmitter further accommodates a device (notshown, either) for actuating the source 1 between operative andinoperative positions, to ensure safe handling of the transmitter incompliance with applicable health standards.

The transducer being described is built into an appropriate assembly 11of a mining machine at a spacing "h" from the medium being monitored,the value of "h" being selected to suit the operation of the machine tobe automated (e.g. a cutter-loader or a winning unit etc.).

The transducer operates, as follows.

Gamma rays from the source 1 fall upon the monitored coal-rock medium Mto be scattered and reflected thereby. Backward scattered gammaradiation is registered by the detector 2. The rays directed straightfrom the source 1 toward the detector 2 are practically completelyabsorbed by the thickness of the materila of the housing 8 of thetransmitter intermediate the source 1 and detector 2.

The intensity I of backward scattered gamma radiation registered by thedetector 2 serves as a measure of the thickness H of the coal band,growing as this thickness grows. Thus, the value of intensity I is takento be representative of the thickness H of the coal band overlying theparent rock, in which way a hidden coal-rock interface is actuallylocated. The detector 2 registers backward scattered gamma radiationpassing through the screen 3, port 9 closer to the source 1 and port 10more remote from this source 1. The material and thickness of the screen3 are selected to satisfy the condition:

    2≦exp (μρd)≦300,

where

μis the mass coefficient of attenutation of gamma radiation by thescreen, cm² /g;

ρ is the density of the material of the screen, g/cm³ ;

d is the thickness of the screen, cm.

With the energy of gamma radiation E=60 keV and the screen 3 made ofiron, the above condition is satisfied by a thickness "d" of the screenfrom 0.1 to 0.6 cm. This range of thicknesses of the screen 3 providesfor obtaining the maximum intensity of backward scattered gammaradiation, registered by the detector 2, which is invariant with respectto fluctuations of the spacing "h" of the detector 2 from the monitoredmedium M. With the thickness of the screen 3 below 0.1 cm the role ofthe screen 3 as a means of separating the reception zones at thedetector 2 is substantially impaired. On the other hand, with thethickness of the screen 3 in excess of 0.6 cm the value of the intensityof backward scattered gamma radiation registered by the detector 2sharply drops. The areas of the square ports 9 and 10 in the screen 3are controlled by varying the length l₁ of the port 9 and the length l₂of the port 10. The areas of the ports 9 and 10 in the screen 3 areselected to provide for the sum of the intensities of backward scatteredgamma radiation received by detector 2 through the respective portsbeing substantially permanent within the predetermined range ofvariation of the air gap "h" between the transmitter and the monitoredcoal-rock medium. This permanence of the summary intensity of bakcwardscattered gamma radiation received by the detector 2 through therespective ports is also ensured by vertically adjusting either thesource 1 or the detector 2, or both.

To provide for adequate performance reliability of the transmitter andto protect the latter positively from mechanical damage, the transduceris mounted on a coal-winning machine with an air gap "h" left between itand the medium being monitored. With the transducer mounted inload-supporting structures (bases of power support units, chutes of aflight conveyer, loading boards and the like), the value of the air gap"h" is set to be from 5 to 10 mm, and invariance of the summaryintensity of backward radiation received by the detector 2 through theports 9 and 10 is provided for within a range of variation of this gap"h" between 5 mm and 60 mm. With the transducer incorporated in thecutting member of a cutter-loader, below the level of the cutting bitholders, the value of the gap "h" is set to about 80 mm, depending onthe radial outreach of the cutting bits employed, and invariance of thesummary intensity of backward scattered gamma radiation received by thedetector 2 through the ports 9 and 10 is provided for within a range ofvariation of this gap "h" from 30 to 120 mm.

Schematically illustrated in FIGS. 5 and 6 is a modified version of thetransducer. It comprises a housing 8 having mounted therein a gamma-raysource 1 set at a spacing B from a detector 2 enclosed in a screen 3.The gamma-ray source 1 and detector 2 are mounted for verticaladjustment. The screen 3 has ports made therethrough, differently spacedfrom the gamma-ray source 1: a port 9 of an area S₁ closer to the source1 and a port 10 of an area S₂ more remote from the source 1, the area S₂of the port 10 remote from the gamma-ray source 1 being greater than thearea S₁ of the port 9 closer to the source 1. One end face of thehousing 8 is cut 12 at an angle α=25°... 50° to the base 13 of thehousing 8, with an opening 14 made in the housing 8 normally to thecutting plane 12 to accommodate the gamma-ray source 1.

With the cut 12 thus made and the gamma-ray source 1 thus arranged,gamma rays from the source 1 are incident on the monitored medium at anacute angle.

The detector 2 in this embodiment of the tranducer is a holder ofgas-discharge counters 15 connected in parallel and mounted normally tothe base 13 of the housing 8.

the operation of the tranducer with the cut 12 and spatial detector 2 issimilar to that of the transducer illustrated in FIG. 3 and describedhereinabove. It should be pointed out, however, that the incidence ofgamma rays at an angle α allows to displace the inversion point of thefunction I_(Z2) =f(a) (curve II in FIG. 2) to a range of spacings A>A₂,thus expanding the range of invariance of the summary intensity (curveIII in FIG. 2) with respect to a varying spacing of the detector 2 fromthe monitored medium M. The expansion of the last-mentioned range isfurther supported by the increased volume of the detector 2 in the formof a holder of gas-discharge counters arranged perpendicularly to thebase of the housing.

The parallel electric connection of the counters steps up the summaryregistered intensity of backward scattered gamma radiation.

For practical use, there have been developed prototypes of radio-isotopecoal-rock interface transducers of three types:

(1) for mounting on conveyer-screw working members (of diameters upwardsof 0.7 m) of coal cutter-loaders, offering invariance with respect tothe air gap between the transducer and the monitored medium varyingwithin a 10 mm to 120 mm range; the overall dimensions of a transducerbeing 110×70×65 mm;

(2) for building into loading boards of cutter-loaders, offeringinvariance with respect to the air gap between the tranducer and themonitored medium varying within a 5 mm to 70 mm range; the transduceroverall dimensions being 130×130×55mm;

(3) for incorporation into the supporting surfaces of the working-faceequipment (bases of power support units, conveyers chutes, supportskids, etc.), offering invariance with respect to the air gap betweentransducer and the monitored medium varying within a 5 mm to 70 mmrange; the transducer overall dimension being 280×120×70 mm.

The prototype transucers employ a source of low-energy gamma radiationof radionuclide of Americium²⁴¹ (radiation energy 60 keV, half-lifeperiod over 400 years, activity 200 mCi), and miniature gas-dischargehalogen counters.

FIG. 7 presents a plot of intensity of backward scattered gammaradiation as a function of the thickness of a coal band overlying parentrock for the three above-described prototypes of compact transducers,the Y-axis being intensity I, pulses per second, and the X-axis beingthickness of the coal band, mm.

The value I_(min) corresponds to the intensity of backward scatteredgamma radiation coming from rock (H=O), and the value I_(max)corresponds to the intensity of gamma radiation scattered backward by a60-mm thick band of coal overlying the rock.

The plot proves that the differentiating (discriminating) capacityδ=I_(max) :I_(min) of the transmitter is at least 2.5:1, and the depthfactor determined at a 0.9 I_(max) level is 45 mm, which is quitesufficient for monitoring a coal-rock interface under conditions offull-seam extraction.

FIG. 8 presents a plot of intensity of registered back scattered gammaradiation as a function of the the air gap between the transducer andmonitored medium for a transmitter mounted on the conveyer-screw workingmembers of coal cutter-loaders; and FIG. 9 presents a similar plot for atransmitter mounted in the loading board of a cutter-loader and othersupporting surfaces of working-face equipment.

The Y-axis shows intensity, pulses per second, and the X-axis is the airgap "h", mm. Curve IV of the plot is a function I=f(h) with gammaradiation scattered by parent rock (H=O); curve V is the same, with acoal band H=10mm overlying the rock; curve VI is the same with a coalband H=20 mm overlying the rock; curve VII is the same with a coal bandH=50 mm overlying the rock. The curves show that the intensity ofbackward scattered gamma radiation varies but slightly and practicallyis permanent throughout the range of variation of the air gap between 10mm and 120 mm (FIG. 8) and between 5 mm and 70 mm (FIG. 9).

The insensibility to fluctuations of the air gap between the transducerand the medium being monitored in combination with the compact size of atransmitter provides for noncontact monitoring of a coal-rock interfacein close proximity to the generatrix of the cutting line and the breast,which enhances the monitoring reliability and the efficiency of theentire system controlling the work-performing members in relation to thecoal-rock interface.

INDUSTRIAL APPLICABILITY

The disclosed method of monitoring a hidden coal-rock interface and atransducer performing this method can be advantageously employed fornon-contact monitoring of a coal-rock interface as part of automaticcontrol of the work-performing cutting members of screw-type coalcutter-loaders, of front-loading units, chamber excavation machines andother equipment operated for full-seam extraction.

The economic effect of the implementation of the invention arises fromreduced ash content of produced coal, from prevention of destruction ofthe rock surrounding a coal seam, and from stepped-up yield of coalowing to the eliminated necessity of maintaining a relatively thicksafety band of coal over the parent rock.

We calim:
 1. A method of monitoring a hidden coal-rock interface, comprising the steps of irradiating the medium being monitored from a gamma-ray source, registering backward scattered radiation by a detector, and determining the hidden coal-rock interface from the intensity of the backward scattered radiation, characterized in that backward scattered radiation is registered at a distance from the surface of the medium being monitored and at a distacne from the gamma-ray source, which does not exceed a preset maximum distance from the detector to the medium, while two zones of reception of backward scattered radiation are formed at the detector, differently spaced from the radiation source, so that the intensity of the backward scattered radiation received by the zone closest to the radiation source diminishes with the distance to the detector to the medium, whereas the intensity of backward scattered radiation received by the zone far from the radiation source increases, in which case the intensities of backward scattered radiation received by both zones are summed up to obtain the total intensity of backward scattered radiation invariant with respect to the varying distacne from the detector to the medium.
 2. A method as claimed in claim 1, characterized in that the invariance of the total intensity of backward scattered radiation with respect to the varying distance between the detector to the medium being monitored, within a preset maximum distance is provided for by pre-changing the surface area of detector reception zones.
 3. A method as claimed in claim 2, characterized in that the invariance of the total intensity of received backward scattered radiation with respect to the varying distance between the detector and medium being monitored is additionally provided for by prechanging the angle of incidence of gamma rays from the source onto the medium being monitored.
 4. A method as claimed in claim 1, characterized in that the invariance of the total intensity of backward scattered radiation with respect to the varying distance between the detector and medium being monitored is additionally provided for by pre-displacing the radiation source and/or detector in the vertical plane.
 5. A tranducer for monitoring the hidden coal-rock interface, realizing the method of claim 1, comprising a housing accommodating a gamma ray source and a detector of backward scattered radiation, whiuch is placed at a distance from the source and shielded by a screen attenuating backward scattered gamma radiation, characterized in that the screen is provided with ports different distnces from the source of gamma radiation, the area of the far port being greater than that of the port closer to the source, while the distance from the gamma radiation source to the detector does not exceed the preset maximum distance from the detector to the medium (M).
 6. A transducer as claimed in claim 5, characterized in that the ports have sufficient area ensuring that the sum of intensities of backward scattered radiation received by the detector through each of the ports is substantially constant within the limits of a present range of the distance from the detector to the medium.
 7. A transducer as claimed in claim 6, characterized in that the material and thickness of the screen are selected from the condition:

    2≦exp (μρd) ≦≠300,

where: μ is the mass coefficient of attentuation of gamma radiation by the screen, cm² /g; ρ is the density of the screen material, g/cm³ ; d is the thickness of the screen, cm.
 8. A transducer as claimed in claim 5, characterized in that one end of the housing has a cut at an angle of 25°-50° to the base of the housing, and an opening is provided in the housing perpendicular to the plane of the cut, this opening accommodating the gamma ray source while the detector is made as a holder with a plurality of gas-discharge counters arranged normally to the base of the housing and connected in parallel with one another.
 9. A transducer as claimed in claim 5, characterized in that the gamma ray source and/or the detector are mounted in the housing adjustably in the vertical plane. 